Perfect engagement

How the wear indicator on a Solo clutch looks when it needs to be replaced.

The clutch has two jobs – allowing smooth application of power from a dead stop and allowing the driveline torque to be easily interrupted and reapplied during shifting. As the clutch wears, it can no longer do these jobs efficiently.

Keeping your manual clutch adjusted – or keeping tabs on wear of self-adjusting clutches – will improve performance, prevent a breakdown and minimize the cost of eventually replacing the clutch.

Why you adjust the clutch
The clutch is held in its normal position – with the driven discs tightly clamped between the flywheel and pressure plate – by powerful springs. As you depress the pedal, you’ll first feel about two inches of free play where nothing happens inside the clutch. As the linkage starts to disengage the clutch through the clutch bearing, you’ll feel the pedal get harder to depress as its action starts to compress those springs and disengage the clutch by pulling the pressure plate away from the flywheel.

As the facing wears off both sides of the driven discs over time, they get thinner and free play is reduced. As wear occurs, the springs must move the pressure plate farther toward the flywheel to clamp them. This moves the bearing closer to the flywheel. As free play disappears, the clutch bearing begins to constantly rest against the clutch fork – a lever that transmits the pedal action to the clutch.

“Once the release bearing contacts the fork, part of the spring load is absorbed by the release bearing,” says Dan Gochenour, customer support manager of the Eaton Clutch Division. “The clutch then can’t transfer torque properly, and the clutch starts to slip.”

As Charlie Auer, director of sales and engineering at ZF Sachs, puts it, “The clutch gets unloaded. Often, this results in slippage and you burn it up.”

Slippage sends the fatigue snowball rolling down the hill because it generates heat, which increases the wear rate “dramatically,” says Gochenour. In fact, if the heat is intense enough, you can ruin the facing so it won’t perform properly even though it may not have worn down. The purpose of adjusting the clutch is to restore the gap that was originally there so the springs can clamp the clutch fully and it won’t slip.

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As a driver, you may notice either or both of two gradually worsening symptoms, according to Gochenour. The first thing you notice may be “release drag. You don’t get clean release, the clutch drags, and you get hard shifting. The other thing is something you may not notice all the time, but on a strong pull the clutch could start to slip. It might only happen once or twice a day at first.”

You can tell the clutch is slipping by listening to engine rpm. You probably have noticed that rpm normally rises and falls steadily with speed once you have your foot off the clutch pedal. When a clutch is slipping, increasing throttle will cause rpm to rise without the truck speed changing, or at least the increase in engine rpm will no longer be in proportion to the change in truck speed.

You can also watch the tach. If doubling the rpm no longer doubles truck speed, the clutch is slipping.

Even though some linkages are adjustable, the critical thing, Gochenour says, is to “be sure to adjust the clutch internally.” He reports that Eaton Easy Pedal clutches have an adjusting ring that’s threaded into the clutch cover. “You turn the adjusting ring to compensate for wear. This will bring the release bearing back to the original position.”

While the ZF-Sachs Twin Extend clutches sold by ArvinMeritor have a different type of spring system, the two clutches function similarly, and their adjusting ring is generally similar, too.

“Adjustment is simple,” Gouchenour says. “The most important thing is to understand what you are doing.” One way to check the clearance is to apply the parking brake and then stand beside the truck with the door open and depress the clutch pedal. You can feel with your hand whether the clearance remains at about 2 inches. Once you sense that the clearance has dropped off, you need to get under the truck and adjust the clutch.

  1. Shut down the engine, apply the parking brake and block the wheels so the truck won’t roll.
  2. The clutch must be fully depressed to remove all the clamp load before you can turn the adjusting ring. Get a friend to help you by depressing the clutch pedal and holding it in. Or, if nobody is available, Auer says you can do what many owner-operators do: Cut a two-by-four to the right length and wedge it between the clutch pedal and the driver’s seat.
  3. Repeat Step 3 until the clearance is 1/2 inch. Then, on the Twin Extend clutch, replace the lockplate and install and tighten its attaching bolt.

When to replace
On a manual clutch, you replace it when it tends to slip on hard pulls or difficult starts.
But both Eaton and ZF Sachs also make self-adjusting clutches. The Eaton version is known as the Solo clutch. Gochenour says it has an arrangement of spring-loaded cams and ramps that senses a change in the bearing travel and adjusts a ring, somewhat like the manual adjustment. Problems with road debris that interfered with smooth automatic adjustment early on were solved initially by installing a plastic cover, and later by casting a protective cover into the clutch housing. The Solo, once set up properly at the factory, never needs adjustment throughout its life.

Auer says the Twin Extend has a similar adjustment mechanism available. But, he says, many owner-operators use the standard model because it saves up-front cost and adjustment is very simple. Many large fleets choose clutches that adjust themselves because they have a hard time keeping track of adjustments and making them in a timely manner.

If you have a self-adjusting clutch, one key maintenance item is watching the wear indicator. You should replace the clutch when the wear indicator reaches the end of its travel in order to avoid slippage, flywheel damage and even potential breakdown.

The Solo wear indicator is a small, black tab located in a slot on the bottom of the housing. The indicator rides counterclockwise around the housing and can be lined up with a red line of varying width in front of it. When the indicator lines up with the thin end of the line and the word “replace,” it’s time to replace the clutch driven discs and inspect and possibly replace the flywheel.

On the Twin Extend, “You look up through the bell housing and look at the indicator. It’s visible through a window,” Auer says. “It migrates as the clutch wears.” You can tell by lining up its position with a scale when it’s time to replace the clutch.

On automated, three-pedal transmissions, the same manual clutches are used as on straight manuals. But on fully-automated, two-pedal gearboxes, there is never a need to adjust the clutch. The Eaton Ultrashift, says Gochenour, uses a clutch that never needs adjustment because it is engaged centrifugally. Weights quickly swing out as you raise rpm from an idle and the mechanism will compensate for wear. Just watch the wear indicator on the housing.

On the Freedomline, Auer says clutch life is extremely long, often more than a million miles, because, “A human can’t engage a clutch like a computer.” The organic-type facing Freedomlines have also lasts longer than the more common cerametallic facing under the ideal conditions of computer operation, where minimal heat is created. In fact, he says, the long clutch life is “part of the cost justification” for this fully automated transmission.

However, the driver must “go by the rules,” which means heeding any warning lights indicating, for example, that he is using the clutch to hold the truck on a hill rather than the brakes, a no-no for any type of clutch. If the light comes on, take your foot off the throttle and begin holding the truck with the brake.

The Freedomline’s clutch does not need any adjustment because it has a diaphragm-type spring that creates a relatively constant spring force with wear until the clutch is just about worn out. When the clutch reaches the point where it’s time to replace or risk operating problems, the driver will see a “CW” light on the shift quadrant at startup, meaning “Clutch Wear.” The computer even keeps track of the stroke of the clutch engagement mechanism. Fleet maintenance people can interrogate with the Transoft scan tool and get a reading of how much clutch life is left.

It is necessary to grease both the shift fork’s bronze bushings and the throwout bearing at oil change intervals with the right grease. Auer says ArvinMeritor has an option that extends the lube interval to 100,000 miles. Gochenour specifies a high-temperature lithium complex grease of good quality. Auer specifies a high-quality chassis grease. Gochenour recommends that you grease the fittings till the old grease is purged and you can see fresh grease coming out.

Clutches are offered as new or remanufactured parts. Quality reman parts are OK if you don’t plan to keep the truck for too long. If you want maximum life, you should specify new parts so factors like spring tension will be optimal.

Remember, too, that the typical cerametallic clutch is hard on the flywheel. If it is not machined or replaced in order to restore the flat, smooth surface, the new driven discs will never last, so make sure the work is done right.

Follow these suggestions about keeping the clutch adjusted, starting up the right way, and replacing the clutch before it can’t carry the load anymore – you’ll increase operating reliability and reduce costs. If you’ve treated the clutch right, it will be also be cheaper to restore the flywheel.

How to Minimize Clutch Wear
The clutch wears during starts but stops wearing altogether once it’s engaged, so one of a driver’s most important responsibilities is to start out properly.

Increasing the slip time by starting out in too high a gear and using too much throttle greatly increases wear, partly because the clutch will run hotter. As Charlier Auer, director of sales and engineering at ZF-Sachs, says, “High rpm and high slip time equal death to a clutch.”

When heavily loaded, especially when on any upgrade, you should start in the No. 1 position on a 9-, 13- or 18-speed, or the No. 2 position on a 10- or 15-speed. An 18-speed’s splitter should be in the lower position, too. With lighter loads on perfectly level roads or slight downgrades you may be able to go up a gear or sometimes even two. Just make sure you can start without a lot of throttle and that the slip time is short.

The sight of a driver trying to start loaded on a hill in too high a gear is all too common. A modern diesel engine will easily do the job if revved up above idle speed because the torque more than doubles from 600 to, say, 1000-1,100 rpm. But there will be a lot of unnecessary wear on the clutch and drivetrain.

On a significant upgrade when heavily loaded, that 10-speed should go into first, and the 9-, 13- or 18-speed should, ideally, be put into low.

Consistently grabbing the right gear for starts, and never slipping the clutch in the upper gears, will reduce clutch wear 80 percent or more compared with what occurs with the most abusive driving.

To reduce wear even more, take maximum advantage of the low-end torque and electronic control of modern engines by “idling away” – not touching the throttle at all when starting out on a level road. Just gradually release the clutch until it begins to grab. The idle governor function of the ECM will increase throttle without allowing an increase in rpm. Unless you release the clutch much too fast, the engine will not stall. This minimizes clutch wear by enabling it to engage at the lowest possible speed. On hills, you can use just a little throttle to increase rpm slightly, but always use as little as possible. Engine clutch engagement torque is usually rated at 800 rpm, and this should be more than enough for even the steepest grade fully loaded – if you’re in the right gear.

Also, when waiting for a traffic light to change, never use the clutch to keep the truck from rolling back. That not only subjects the clutch to the wear of a number of starts but heats it up, which increases that wear by several times. Heed warning lights about such usage when running an automated transmission.

For more information:
(248) 435-1000

Eaton Corp.
(800) 826-1000